Pacific Coast: San Juan River Delta

The San Juan River delta coastline (Figs. 15 and 16) is approximately 50 km long and is characterized by the alternation of barrier islands, 8 to 12 km long, interrupted by "funnel-shaped" estuarine tidal inlets (West 1957, Martínez et al. 1995; Correa 1996). The San Juan delta has an area of 800 km2, a drainage basin of 16,500 km2, and the highest discharge (2550 m/s), sediment load (16,000,000 t/yr), and basin-wide sediment yield (1150 t/km2/yr) on the west coast of South America (Restrepo et al. 2002).

Figure 15. (left) Radar image of the San Juan River delta and the northwestern part of Málaga Bay. Note the four barrier islands fronting the delta, interrupted by "funnel shaped" estuaries of the main distributaries of the San Juan River. Tertiary rocks flank the delta and also crop out within the delta plain. Image was modified to show the approximate location of breaching of the El Choncho barrier island (Ch B. I.). Reproduced by permission of Instituto Geográfico Agustín Coddazzi (IGAC). [larger version]

Figure 16. (right) Geomorphic classification of the southeastern part of the San Juan delta, prior to breaching of the Isla El Choncho barrier spit. Note the ancient Santa Bárbara beach-ridge system (dated at 1320 ±75 B.P.), and extension of the sandy tidal flats at Boca Chavica and Boca San Juan. Modified from Correa and Gonzalez (2000). [larger version]

The San Juan delta coastline is essentially unstable, especially along its northern and southern extremes (Boca Togoromá Charambirá and Boca Chavica) where beach retreat (Fig. 17) has been about 100 m in the last 50 years (Correa and Restrepo 2002). The historical evolution of El Choncho barrier island, at the southern part of the delta, shows a general pattern of barrier narrowing and breaching (Fig. 18) that is becoming common along other barrier islands of the Pacific coast, including Charambirá (northern part of the delta), and along the Patía delta and Mira deltas to the south. According to radiocarbon dates, the Isla El Choncho barrier island began forming about 500 yB.P., and grew to the southeast by successive beach-ridge accretion (González and Correa 2001). Its evolution in the last 50 years records a succession of events that segmented the island. The general retreat of the central part of the barrier (Fig. 18) coincided with the formation of an extensive sandy tidal flat at its northern end (Boca Chavica), which diminished sand supply from the north. Shore erosion was accelerated after a 20 to 30 cm co-seismic subsidence event generated by the November 19, 1991 (M 6) earthquake. After the 1991 subsidence, the barrier flooded from 2 to 17 times each year during highest spring tides.

Final breaching of the barrier (Fig. 18) occurred in 1997 and coincided with several overwash events (Fig. 19) associated with positive sea-level anomalies of 30 cm caused by El Niño (Morton et al. 2000, Morton et al. 2002). Following a prudent strategy, El Choncho village (Fig. 20) was relocated to the Santa Bárbara beaches, a beach-ridge remnant located 200 m inland from the shoreline (Correa and González 2000).

Figure 17. Erosional beach along El Choncho barrier island, just north of the breach. Trees on the beach are typical of the supratidal, freshwater zones of the barriers. Photo by I. Correa.

Figure 18. Central part of El Choncho barrier island after breaching of the new inlet, near El Choncho village. Photo by I. Correa.